The invention relates to a fluid friction coupling. The coupling includes a hub, a housing and at least two sets of plates. An interior cavity is formed between the coupling housing and the hub and the portion not occupied by the plates is at least partially filled by a viscous fluid The plates of one set and the plates of the other set are alternately interleaved with one another. The plates are non-rotatingly associated with the hub and housing. The plates of one set of plates include radially extending slots which open towards the plate edge. Plate segments are formed between the slots and include scraping, radially extending chamfered portions which may contact the surface of one of the opposed planar plates The chamfered portions project axially from one side of the plates planar surface.
Prior art fluid friction couplings have their coupling plates alternately arranged such that one coupling plate is associated with the one coupling part and the alternating plate coupled with the other coupling part. Ordinarily, the outer plates associated with the housing are preferably arranged at a desired distance from one another. Relative to the housing outer plates, the inner plates are axially movably supported in hub teeth which non-rotatingly accommodate them. The interior cavity of the coupling, which houses the plates, is at least partially filled with silicon oil. These couplings are preferably used in drivelines of motor vehicles For example, the couplings are used in four wheel drive vehicles, between the rear and front axle, in connection with a differential to ensure that if a low adhesion coefficient occurs at the front or rear wheels, there is a differential effect by connecting the drive with the wheels of the other axle.
If a speed differential occurs between the coupling housing and hub, e.g. between the outer plates and inner plates, the shear load affecting the viscous medium, e.g. silicon oil, causes a moment build so that a torque is transmitted to the normally non-driven wheels. The shear load acting on the viscous fluid results in a temperature increase and thus in an expansion of the volume of fluid in the interior cavity of the fluid friction coupling. As soon as the interior cavity is completely filled, the unfixed inner plates begin to move axially in the direction of one of the adjoining fixed outer plates. This movement, caused by the normally movable plates not remaining centrally positioned between two fixed plates, forms different gap sizes between the adjoining plates. Because of different throttle effects on the two sides of the plate, the pressure increases more quickly in the larger gap so that the movable plates move in the direction of the fixed plate adjoining the smaller gap. When the movable plate comes into contact with one side of the fixed plate, a friction locking effect occurs so that the coupling operates at least in a dual mode. On the one hand, the viscous fluid is subjected to a shear load and on the other hand, there exists a friction contact. Thus, the coupling acts as a friction coupling because the plates are in contact with each other. This effect is referred to as "hump" effect. Hump effect constitutes a kind of self-protecting function for the coupling since it prevents overloading. The hump mode of a fluid friction coupling is achieved for example if there exist different friction conditions between the individual vehicle wheels and the ground so that there may occur a differential speed between the wheels of one axle or between the wheels of two axles.
The transition of the fluid friction coupling into the hum mode may be initiated prematurely by suitable supportive measures. DE-PS 38 28 421 C 1, DE-PS 38 28 422 C 1 and DE 39 34 327 C 1 each propose a fluid friction coupling with a set of slotted plates. The radially extending slots form segments, between adjacent slots, whose radially extending edges are bent. This design improves the transition from a normal operating mode into the hump mode as a result of the axial force component caused by the hydrodynamic effect of such bent portions.
The disadvantage of above fluid friction coupling plates is that a reaction force occurs which acts o the plates against the axial force component, which acts on the viscous fluid, and results in a pumping effect which presses the fluid contained in the coupling interior against one axial end of the housing. In extreme cases, this could lead to the plates arranged at the opposite end of the coupling interior cavity to run dry, causing increased plate wear.